Is UK obesity fuelling an increase in 10 cancers?

“Being overweight and obese puts people at greater risk of developing 10 of the most common cancers,” reports BBC News.

The news is based on research using information in UK GP records for more than 5 million people, to see whether body mass index (BMI) was associated with 22 types of common cancers.

The researchers found that increasing BMI was associated with increased risk of several types of cancer. Some of these associations weren’t linear, meaning that there wasn’t always a steady increase in cancer risk with increased BMI. Additionally, some of the links seemed to be dependent on individual patient characteristics, such as gender and menopausal status.

The researchers estimated that 41% of uterine and 10% or more of gallbladder, kidney, liver and colon cancers could be attributable to excess weight.

However, increasing BMI was also found to decrease the risk of some types of cancer (such as prostate and premenopausal breast cancer).

The researchers suggest that BMI affects cancer risk through a number of different processes. However, the study was not able to demonstrate that being overweight or obese directly increase or decrease risk of these cancers, nor is it able to show the biological reasons for any of the associations found.

It is also not able to account for all possible factors that contribute to cancer risk, such as genetics and lifestyle factors.

Nevertheless, maintaining a healthy weight has proven benefits beyond any reduction in cancer risk. As always, the best way to do this is by eating a balanced diet and exercising regularly.

Where did the story come from?

The study was carried out by researchers from the London School of Hygiene and Tropical Medicine, and the Farr Institute of Health Informatics Research. The study was funded by the National Institute for Health Research, the Wellcome Trust and the Medical Research Council.

The study was published in the peer-reviewed medical journal The Lancet. This article is open-access and can be accessed for free on the journal’s website.

The story was widely covered by the media.

What kind of research was this?

This was a cohort study that aimed to investigate the link between BMI and the most common site-specific cancers after adjusting for potential confounders.

As this is a cohort study, it cannot prove that obesity causes cancer, as there may be a wide variety of other factors (such as hereditary, sociodemographic and lifestyle factors) that could explain the associations seen.

What did the research involve?

The researchers studied primary care (GP) records from 5.24 million people, using data collected between 1987 and 2012.

They calculated BMI from recorded weight and height, both of which are recorded by GPs when patients are registered, during patient care, or because the GP thinks it’s relevant to the patients’ health.

The researchers then looked to see if people had a cancer diagnosis in their records, in particular:

female breast cancer

prostate cancer

mouth, oesophageal, stomach, colon and rectum cancers

lung cancer

non-Hodgkin lymphoma

leukaemia and multiple myeloma (blood cancers)

ovary, uterus (womb) and cervix cancers

pancreas, brain and central nervous system cancers

liver and gallbladder cancer

kidney and bladder cancer

thyroid cancer

malignant melanoma

The researchers looked to see whether BMI was linked with increased risk of cancer. They estimated the average effect of a 5kg/m² increase in BMI on cancer risk.

They controlled for age, smoking status, alcohol use, previous diabetes diagnosis, socioeconomic status, time period and gender in their analyses.

What were the basic results?

People were followed for 7.5 years on average, and during the study, 166,995 people (3.2%) developed one of the cancers of interest.

The researchers found that a 5kg/m² increase in BMI was associated with an increased risk of the following types of cancer:

The researchers noted that not all the associations were linear, and that the associations between BMI and both colon and liver cancer were more marked in men than in women. Increases in ovarian cancer risk with BMI were larger in premenopausal than postmenopausal women, and there were differences by menopausal status for breast cancer.

The researchers estimated that 41% of uterine and 10% or more of gallbladder, kidney, liver and colon cancers could be attributable to excess weight.

A 5kg/m² increase in BMI was associated with a reduced risk of the following types of cancer:

premenopausal breast cancer risk (HR 0.89, 99% CI 0.86 to 0.92)

oral cavity (HR 0.81, 99% CI 0.74 to 0.89)

lung (HR 0.82. 99% CI 0.81 to 0.84)

There was a borderline statistically significant reduction in the risk of prostate cancer (HR 0.98, 99% CI 0.95 to 1.00).

The researchers noted that when the analysis was restricted to people who had never smoked, a 5kg/m² increase in BMI did not reduce the risk of oral cavity or lung cancer. They suggest that this inverse association seen when all people were considered was due to residual confounding.

How did the researchers interpret the results?

The researchers concluded that, “BMI is associated with cancer risk, with substantial population-level effects. The heterogeneity in the effects suggests that different mechanisms are associated with different cancer sites and different patient subgroups.”

Conclusion

This large UK cohort study of more than 5 million people has found that, although there was variation in the effect of BMI on different cancers, a higher BMI was associated with increased risk of several cancers.

Overall, the researchers estimated that a 1kg/m² population-wide increase in BMI would result in 3,790 additional people in the UK each year developing uterus, gallbladder, kidney, cervix, thyroid, leukaemia, liver, colon, ovarian or postmenopausal breast cancer.

However, not all of the identified links were completely clear, with some showing a clearer linear association between increasing BMI and increasing cancer risk than others. Also, strangely, increased BMI was also found to decrease the risk of some types of cancer, such as lung cancer. Such associations may be explained by other factors: for example, smokers – who are obviously at a much higher risk of lung cancer – tend to have a lower BMI than non-smokers.

However, this study is unable to demonstrate that being overweight or obese definitely directly increase or decrease the risk of these cancers. The researchers suggest that BMI affects cancer risk through a number of different processes. The study is also not able to account for all possible factors that may be entangled in the links (such as various hereditary, sociodemographic and lifestyle factors).

Nevertheless, it is well established that maintaining a healthy weight has many health benefits, including reducing the risk of many common chronic diseases. The best way to do this is by eating a balanced diet and exercising regularly.